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look at boiling points -
ATE is ... 500
Castrol SRF is ... 590
Motul 600 is ... 594
Motul 660 is ... 622
soft pedal is supposed to be from brake fluid near the caliper getting boiled by the high temp of the rotor. boiling makes little air bubbles. air is compressible. are you getting little bubbles at the caliper when you bleed? further, the brake fluids with high boiling points tend to be hydroscopic - they attract water. water lowers boiling point and makes fluid more susceptible to getting boiled. hence the need to bleed the high-end fluids more frequently. and if even the best fluid continues to boil, then perhaps you need better airflow, or new rotors (less rotor material means less heat mgmt ability). ditto pads. is it the front or back that is starting to cook-off - bet you've got no cooling on the back ... i've heard of 987's needing rear cooling because the pasm works the rears more than in a standard system.
if no little bubbles then the soft pedal is from something else. cheap pads off gassing? had that happen once. once. change in performance of abs as tires heat up? i've often thought that the chinzy way the car makes vacuum for the brake booster has something to do with it, and as the engine works harder, vacuum and boost increase? there's a reason porsche went to the tandem pump in the 9x7.
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